Gases such as natural gas, flue gas and synthesis gas have been purified by the utilization of aqueous alkanolamine solutions for the adsorption of acid gases such as CO.sub.2, H.sub.2 S and COS contained in the gas stream. The alkanolamine solution (e.g., a monoethanolamine or diethanolamine aqueous solution) flows counter current to the gas stream in an absorption column in order to remove the acid gases. An advantage of such a system is that the process is a continuous cyclic one and the reaction can thus be reversed at higher temperatures in order to regenerate the alkanolamine from the rich solution.
When steel parts or components are used in such a system, it has been found that both general and local corrosive attack can occur. This is a particular problem in reboilers and heat exchangers where the steel is exposed to a hot, protonated alkanolamine solution. A heat transferring metal surface appears to be especially vulnerable. Previous investigation by others have revealed that under certain conditions corrosive products such as aminoacetic, glycolic, oxalic and formic acids were formed. The monoethanolamine or diethanolamine salts of these acids present the possibility of increased attack upon ferrous metals. Furthermore, the carbonate salt of monoethanolamine can be converted to additional products such as N(2-hydroxyethyl) ethylenediamine which has been found to increase corrosivity towards steel, particularly under heat transfer conditions.
The use of corrosion inhibitors to inhibit corrosion of metallic surfaces in contact with aqueous alkanolamine solutions is well known. In U.S. Pat. No. 3,808,140 a corrosion inhibiting composition is taught based on the synergy of using a combination of antimony compounds and vanadium compounds. The patent also acknowledges the independent use of antimony compounds and vanadates separate from their combination.
Another problem in the use of monoethanolamine for gas treatment relates to its instability in solution with time. Degraded monoethanolamine has to be replaced frequently with the attendant cost of fresh monoethanolamine solutions and the time lost by the shutdown of the absorption regeneration process.
To combat degradation, trialkanolamines, such as triethanolamine, triisopropanolamine, and triisobutanolamine, are introduced into the monoethanolamine solution, as taught in U S. Pat. No. 3,535,260, which is incorporated herein by reference. The trialkanolamines, however, suffer from the disadvantage of having a much higher boiling point and a higher molecular weight than monoethanolamine thus they cannot be recovered during the regeneration step.